This invention relates to a liquid filter for cleaning the lubricating oil of an internal combustion engine with a filter housing closed by a removable cover; an annular filter insert which is removable for maintenance, through which liquid to be filtered flows radially, and which separates a dirty liquid chamber from a filtrate chamber in the filter housing; a dirty liquid passageway for delivering liquid to be filtered; a drain passageway leading from the filter housing to a liquid collecting container, and a shut-off device associated with the drain passageway for opening the drain passageway when the filter housing cover is loosened and the filter insert is removed, the shut-off device comprising a first valve which shuts off the drain passageway via a spring when the filter housing cover is closed and the filter insert is installed.
With liquid filters of this kind the spilling of the liquid when the filter housing cover is removed is prevented by the fact that the filter insert, which is held in its installed position by the cover, is moved outwardly with the cover and a shut-off device automatically opens the drain passageway so that the liquid situated in the filter housing can flow down into a liquid collecting container.
Published German Patent Application No. DE-OS 34 22 482 discloses such a lubricating oil filter. A shut-off device is integrated into the filter housing. This shut-off device is a combination valve comprising a shut-off valve and a bypass valve. It has been found that such a shut-off device, which consists of a plurality of valve parts, springs etc., is rather difficult to install in the filter housing. Furthermore, a number of sealing surfaces have to be provided in the filter housing and must cooperate with the shut-off device. All in all, therefore, this known combination valve disposed in the liquid filter is an apparatus which is difficult to manufacture.
Published European Patent Application No. EP 314,915 discloses a liquid filter which likewise opens up a passageway to drain the liquid when the filter housing is opened. This drain passageway can be closed by a valve body which is supported by two inserted compression springs. This approach is less costly, yet with this arrangement there is the danger that a spring may stick so that a good seal may not be assured when the filter insert is installed. Faulty sealing of the drain passageway can result in an inadequate supply of oil to the motor.
Another disadvantage of this arrangement is to be seen in the fact that a precise matching of the springs is necessary if the valve body is to perform its task. Furthermore, the filter housing must here again be provided with sealing and attachment surfaces, thereby increasing the cost of production.